1. Field of the Invention
The present invention relates to a DC/DC converter for dropping an external power supply voltage to be input by turning a switching transistor ON/OFF so as to acquire a predetermined DC output voltage.
2. Description of the Related Art
One type of a DC/DC converter uses a method of installing a switching transistor and dropping an external power supply voltage by turning the switching transistor ON/OFF so as to output a predetermined DC voltage. This switching type DC/DC converter has been widely used since it is compact and is highly efficient (e.g., Japanese Patent Application Laid-Open No. H7-336999, Japanese Patent Application Laid-Open No. H11-332222). Generally, for a switching type DC/DC converter, a linear regulator for outputting an internal power supply voltage, with which normal operation of a switching control circuit for controlling the switching transistor is guaranteed, is used (e.g., U.S. Pat. No. 5,528,132).
FIG. 2 shows an example of a conventional DC/DC converter. In this DC/DC converter 101, the switching transistor 111 turns ON/OFF according to the switching control signal which is output by the switching control circuit 113, so that the external power supply voltage VIN to be input is dropped through the subsequent smoothing circuit 112, and DC output voltage VOUT is output from the DC output terminal OUT. The switching control circuit 113 feeds back and inputs the DC output voltage VOUT, and controls the switching transistor 111 based on this input. The external power supply voltage VIN is input to the linear regulator 116, and in the ON status (unless the later mentioned comparison circuit 114 changes the status to OFF), the linear regulator 116 supplies a constant DC voltage to the switching control circuit 113 as the internal power supply voltage VDD. The comparison circuit 114 compares the DC output voltage VOUT with the reference voltage VREF, and turns the switch 115 OFF and turns the linear regulator 1160N if the DC output voltage VOUT is lower than the reference voltage VREF. If the DC output voltage VOUT is higher than the reference voltage VREF, the comparison circuit 114 turns the switch 1150N and turns the linear regulator 116 OFF.
When this DC/DC converter 101 is started up at the rise of the external power supply voltage VIN, the internal power supply voltage VDD is supplied from the linear regulator 116 since the linear regulator 116 is ON. When the DC output voltage VOUT rises and becomes higher than the reference voltage VREF, the switch 115 turns ON and the linear regulator 116 turns OFF by the output of the comparison circuit 114. Then the DC output voltage VOUT is supplied as the internal power supply voltage VDD. Also, the DC output voltage VOUT rises up to the DC output set voltage and stabilizes, that is, it enters normal operation status.
If the total current flowing through the switching control circuit 113 is Is (e.g., about 10 mA), and if the linear regulator 116 is in ON status (at startup), the power consumed by the linear regulator 116 and the switching control circuit 113 is VIN×Is (e.g., about 200 mW if VIN is about 20V). If the linear regulator 116 is in OFF status (during normal operation), the power consumed by the linear regulator 116 is 0, and the power consumed by the switching control circuit 113 is VOUT×Is (e.g., about 55 mW if VOUT is about 5.5V). The DC output voltage VOUT is lower than the external power supply voltage VIN, so the power consumption during normal operation is decreased.
However in this DC/DC converter 101, where the DC output set voltage is relatively high (e.g., about 7V), this voltage is applied to the switching control circuit 113 as the internal power supply voltage VDD during normal operation. If the switching control circuit 113 is integrated in a semiconductor integrated circuit, and if the semiconductor integrated circuit is manufactured in an ordinary fabrication process, that is a fabrication process with a low voltage-resistance, the power supply voltage that can be applied thereto is low (e.g., about 5.5V), which is even lower if a fine process is used. Therefore, the DC output set voltage must be low to fabricate a semiconductor integrated circuit using an ordinary fabrication process. In order to set the DC output set voltage relatively high, a semiconductor integrated circuit integrating a switching control circuit 113 with many elements must be fabricated using a process with a high voltage-resistance, but manufacturing cost increases since the chip size and process cost increase.